The command switch may, for example, be an EMERGENCY-OFF switch which has a red cap in the form of a pot or mushroom-head. The cap is normally firmly screwed to the plunger. When the command switch is operated, then the plunger of the actuator is moved from the rest position to the operated position. During the process, the plunger necessarily interrupts the contacts of the break switching element, so that the circuit, for example of a connected electrical machine, is broken. Furthermore, depending on the application, further break and make switching elements can be accommodated in the contact maker and are operated jointly via the plunger.
European Standard EN 418 “The Safety of Machines”, issued in 1992, also specifies the safety requirements for EMERGENCY-OFF devices. In this context, paragraph 4.1.2 states that the command device and its actuating part must operate on the principle of positive operation. A switch with positive opening is, for example, a suitable command device.
The status of the break switching element can alternatively also be checked by means of a monitoring device in which, for example, a bit pattern is injected via the closed break contacts of the break switching element, and the received result is checked continuously. One such monitoring device is, for example the ASI-F monitoring device from the Siemens Company. This device is also bus compatible, so that, in the absence of the expected bit pattern, installation parts which are accessible for data purposes via a bus can be switched to a safe state.
Furthermore, the command switch is formed in two parts. The capability to separate them is necessary in order to allow the command switch to be attached by way of the actuator, for example to a switching cabinet door, a front panel or a switchboard. By way of example, for this purpose, the actuator is passed through a relatively small hole in a switching cabinet door or the like for installation, and is firmly connected thereto by way of an attachment nut or a clamping ring. The contact maker can then be attached to the actuator, normally by way of a snap-action or screw connection. Thus, the live parts of the command switch are also accommodated, for example, in the switching cabinet.
If the contact maker is not correctly attached to the actuator, for example because the contact maker has not been snapped in correctly, or the screw connection has become released because of vibration, then the contact maker may become detached from the actuator, without this being noticed. An EMERGENCY-OFF circuit then still remains closed, despite the EMERGENCY-OFF switch being operated. The EMERGENCY-OFF switch therefore loses its functionality, and therefore its protective function. This can lead to unpredictable damage to people and installation parts in controlled installations.
DE 41 01 493 A1 discloses, for example, a command switch or an EMERGENCY-OFF switch which allows the function of the contact maker to be monitored.
This has the disadvantage that the contacts of a make contact provided there for monitoring a contact maker may become welded or stuck, so that the fact that the contact maker has become detached from the actuator is not noticed, since the contacts of the make contact still remain closed.
A further disadvantage is that a contact spring which is provided for the make contact and holds the make contact open in the inactive state may break. In this case as well, the fact that the contact maker has become detached from the actuator will not be evident.